Method for the improvement of areal sweep during secondary recovery



Jan- 22, 1953 B. HABERMANN I METHOD PoR THE IMPROVEMENT 0E AREAL swEEPDURING SECONDARY RECOVERY med sept. 2s, 1959 3 Sheets-Sheet l afMz/cwHAJERAMNA/ A er gli A l Jan. 22, 1963 B. HABERMANN METHOD FOR THEIMPROVEMENT OF AREAL SHEEP DURING SECONDARY RECOVERY s sheets-511581 2Filed Sept.' 25, 1959 Erfra- INVENTOR. ENZ/0N HAaERMA/VA/ ,Jj MW",4rromvfy Jan. 22, 1963- a. HABRMANN 3,074,481

METHQD FOR THE IMPROVEMENT OF REAL SWEEP Filed Sept. 25. 1959 DURINGSECONDARY RECOVERY 3 Sheets-Sheet 3 l r INVEVTOR.

- 'Nz/QA/ HA ERMANA/ This invention relates to secondary recovery ofpetroleum from subterranean reservoirs. Specifically, this inventionrelates to a method for obtaining a high sweep efficiency of adisplacing fluid through the reservoir.

In secondary recovery of petroleum from a reservoir,

a Widely accepted practicehas been the injection of a fluid into thereservoir under suliicient pressure to force it through the reservoir toa producing well, thereby sweeping oil from the reservoir. The choice ofinjection and withdrawal wells is widely varied and depends on the typeof secondary'recovery process as well as the geometry of l United StatesPatent O the reservoir. Among the techniques which have been employedare pattern drives, eg., 5 spot, 'L spot, etc., which are commonly usedwith a liquid drive fluid such as water, and line or staggered linedrives which are commonly employed with a gaseous drive fluid. Othertechniques, such as edge and crestal injection are used when needed tosuit the reservoir geometry.

A widevariety of fluids has been used or proposed for use as thedisplacing fluid, and these fluids may be con` venicntly grouped into agas, liquid, or combined gasliquid classification; the term gas" as usedherein being generic to those materials which normally exist in the gasphase under atmospheric conditions as well as those fluids which eXistinthe vapor state at the reservoir conditions. Examples of gas drivefluids are air, nitrogen, carbon dioxide, and various hydrocarbon gases,such as methane, ethane, and natural gas. Liquid'displacement fluids,i.e., materials which are liquid or are in a critical phase above boththeir critical temperature and pressure at reservoir conditions, can begrouped into aqueous and non-aqueous liquids, examples of the latterincluding hydrocarbons such as liqueed petroleum gas (LPG), propane,butane, isobutane, pentane, isopentane, hexane, isohexane, crude oil,kerosene, gasoline, petroleum ether, xylene, toluene, benzene, coal taroils, etc.; oxygenated compounds such as glycerol, phenol, methyl,ethyl, propyl, and isopropyl alcohols acetone, acetaldchyde, propanol,etc. Examples of suitable aqueous drive fluids are water, brine, andaqueous solutions of various wetting or surface active agents and/orthickening agents, eg.. fatty acid soaps, alkyl pyridinium compounds.metal alginates, sodium lauryl sulfate, etc., as well as wafer solublepolymers such as polyvinyl or polyallyl alcohols, carboxymethylcellulose. and partially hydrolyzed polyacrylamide. Because of theirmiscibility with crude oil, hydrocarbon drive fluids provide a morecomplete displacement of the reservoir oil than is obtainable withaqueous solutions; however, their relatively high cost offsets theirefficient displacement,

and water, with or without wetting, surface active, or

- thickening agents, is predominantly employed as a liquid drive uid.

Combined gas-liquid and miscible liquid systems have also been used orsuggested for use. Among such systems is the mixed gas and water phasedrive which is achieved by the simultaneous injection of water andnatural gas into a reservoir, and the various gas-hydrocarbon liquiddrives which have recently received much attention. The latterdisplacement, frequently referred to as miseible phase or miscible slugdisplacement, nvolves first injecting into the reservoir a hydrocarbonwhich is either a liquid or in a critical phase above its criticaltemperature and pressure at the reservoir condi- Patented Jan. 22, 1953ice tions and which is miscible with'the reservoir oil, and then drivingthe injected hydrocarbon slug through the reservoir with a gas or acombined gas-water mixture which is also miscible with the liquid slug.A modification of. this process employs a slug with a gradatedconcentration of hydrocarbon liquid and drive gas to avoid a sharp?"interface between the drive gas phase and the slug phase.

Hydrocarbon liquids employed in these processes have v been LPG, butane,crude oil, and petroleum distillates.

A third type of miscible fluid displacement process herein considered asa liquid drive system is the high pressure condensing gas method whereina gas, such as natural gas, is injected at pressures in excess of about3,000 p.s.i., into the reservoir. The advancing front of gas acts as asolvent to dissolve light fractions of the reservoir oil and therebyform, in situ, a liquid slug phase which is between the remaining gasdrive and the reservoir oil and is miscible with each.

Another type of miscible fluid displacement involves the injection of aliquid which is miscible with the reservoir oil and with water which isemployed as the drive fluid. Various alcohols, such as methyl, ethyl,propyl, isopropyl, and butyl alcohol, phenol, glycerol, etc.; ketones,such as acetone, butanone, etc.; aldehydes, such as acetaldchyde,propionaldehyde, ete; and various other solvents such as carbontetrachloride. methyl chloride, dimethylether, sulfur dioxide, anddichloro-difiuoromethane can suitably be used. Because many of thesesolvents freA quently have a viscosity which is less than that of thelFIGURE 2 is a map of actual tlow conditions of a fluid displacementprocess in a laboratory model as described in Example l,

FIGURES 3, 4 and 5 are maps of actual flow conditions of the fluiddisplacement process with application of the invention as described inExample II, and

FIGURE 6 illustrates a ilow pattern typical of the in',- ventiori asapplied to a line drive fluid displacement procL ess. v

Common to all" these types of secondary recovery is the problem ofobtaining satisfactory distribution of the injected fluid throughout thereservoir, since the latter is not a homogenous mass having the samestructure throughout, but, rather, contains zones having differentpermeabilities and fissures, cracks, or channels having relatively highpermeability to the injected fluid. These irregu larities tend to directthe flow of the injected fluid into narrow lingers or channels throughthe reservoir rather than as a smoothly advancing front, lay-passingmuchof the reservoir oil which is never recovered. This fingering tendencyof the injected fluid is also caused by the indiscriminate choice offluids having very low viscosities relative to the reservoir oil sinceit is related to the mobility below with reference to the.

points-7, 8 and 9 on the central circular arc.

on the advancing front lies directly between injection', well 2 andproduction well 5 and is closer to well 5 thany 3 where as'.a1=viscosity of displacing huid and displaced lud,

respectively; and Kg, Kl=perrncability of the reservoir with respect tothe displacing lluid and displaced liuid, respectively;

circular. arcs'around injection wells 1, 2 and3 and in its finalposition by arcs'a and-bv between wells 4-,and5 and Sjand 6, while theshadedfarea representsthat portion v of thev reservoir not swept by theinjected uid.' TheV reasonfor this anomalous iow distribution isapparent` from ,aA study of the geometry of the system -shownby Point 8,

any other pointon the .fluid front, Such as 7 and 9. The

I'resistance to iluid how from point ,8 to well 5 is proportionalto-distance d which can be seen to be less than" distances c or e forpoints 7 and 9, respecntively.v Accordingly, point 8 advances ,morerapidly to well 5 than do points7 or-9,v resultingA in the final irontassumingk the shape-:shown by lines a and b.

Fromthexforegoingyit is apparent' that satisfactory flowr distributionof a displacement fluid..through a reservoir is di'lcult to achieve .andthat, even with additiorrof viscous additives to thefdisplacing iluidtoachieve favorable mobility ratios; a complete-areal sweep vof the. i

il'uidfis. not. possible with customary secondary recovery techniques..

- ItI is purpose of 'this invention to improvethefsween efficiency insecondary -lrecoveryf by theA iluid'fdisplacement rnethod;v v

'This purpose'. is achieved-"b-yf thenovel injection and:

displacement method of' this invention which, in. itsv lluid` intothe'reservoir through a first. well' communicate .ing` with:thefreservoir. The displacement fluid can. be. anyof thenurnerousaforementioned -lluids or combina.-

tions tlziereof-whichy have been. used oit-suggested forlise.

, simplest form; comprises: first; injecting a displacement Land'various other solvents such. as acetonitrile, acrylonis front itdoes not slow down this portion. 'The cilcct of this injection,therefore, is to cvcn thc 'front by increasing the rate of advance ofthe trailing portions relative to the elongated portions. .The viscosityand quantity of the viscous lluid which is injected into the second wellcan be readily controlled toproduce a zone of any volume and viscosity,lfor instance, a relatively expanded zone o moderate viscosity, or aconcentrated zone with a high viscosity dependingxon the'size and degreeof elongation ofthe leading portions lof the front. As the frontadvances, the injected viscous liquidtends to distribute itself acrossthe'frontwith the greatest conv centration at the most .-forward'pointof the front as it is slowly, forced towards the producing well. VAftersucient viscous liquid has been added to retard lingering v of thedisplacing fluid, the second well can be closed olf or alternativelyswitched to injection of the displacing huid in combination with, or insubstitution for, the injectionvo the 4displacing fluid through the rstwell.- After-the fluid front has reached the third well and againbecomes deformed with elongated-portions at each `well site, theprocedure lis repeated anda fourth row of 'wells is put on production towithdraw the displaced oil. This procedure is repeated as many times asmay he desired' or as may be necessaryl to'sweep the entire reservoir. j

The viscous liquid'which isinjected into the reservoir can suitablycomprise any cfa number of vscous liquidswhich do not plugor seal olfthe formation and which are more viscous than eitherithe reservoiroil orthe displacement duid: Generally, a liquid isemployed which has aviscosity 11A. to 20 times greater than either the reservoir oil ordisplacement iluid viscosity, whichever is greater, and preferablya/liquid which has a viscosity Z'to 10 times greater.

Liquids suitable foruse asl the invention can be for instance; water;brine; hydrocarbons, such as aromatics, e.g., benzene, toluene, xylene,`etc., crude oil and fractions thereof, e.g'.,'.distl1ates, reducedcrude, tar, pitches, and waxes (the latter substances .aire to be usedwith caution toavoid plugging the formationy.v oxygenated compounds,lsuch as -.alcohols,.e.g., methyl,l

ethyl, normal and iso propyl, butyl, arnyl alcohol, phenol, glycol,glycerol, etc., ethers, eg., methy ethyl ether, d-

methyl diethyl ether, etc., ketones, eg., acetone, butanone,

etc'., and aldehydes, e.g., acetaldehyde, propionaldehydei A tlrile,lcarbon tetrachloride,v methyl. chloride, sulfurdisecondaryreeovery;Concurrently witlrzthisv 1'rtjec:l

tiontorf'if ldesired at'aclater timeafter the reservoirhas been"pressurized, withdrawal of oil is initiated from a.

seconda well spaced apar-tf from said. firstwells' "Ihese:v yvellswillvusually, but` not necessarily, be;well`s which# werel drilledjduring.theA previous primaryl production fromI thev reservoir;v For the idealcase withha mobility ratio-of the displacing .uidl to the oil.. phase-ofone'or less and withy a reservoi'rfree from zoneshaving-,zdiler'fentpermeabilitiesvand'-extensiveA fracturesor fissures.

thel displacingfluid willcadvance-uniormly as a smooth.

.front until it nears thegwithdrawal well where the local--izedwithdrawal of the'oil at the Well will elongate that portion or' thefrontA lying directly between thewithdrawal andA injection well. At thispoint in the operation, production: from -the second well is stopped,land a viscous uid. is-.injected into .the reservoirthrough the secondwell yand-,withdrawal of'the oilfrom the'reservoir is initiated; at athird well spaced apart from said second well and so located thatthe'displacing fluid will continue 1 toi'm'ove inthe same generaldirection. The, viscous ,liquidA is thus injected into. the reservoir atthe most extendedor elongated portion of'the advancing fluid front.'vBecause of its high viscosity, this liquid elec,

tively slows-or retards the` advance of the leading orv elongated regionof the iront, but since the viscous liquid is notinthe ow path of thetrailing portion of the oxide; and dichiaro-diuoromethane. The` purposeof the invention can be achievedwhethet" or not` the viscous liquid islmisciblewith. the reservoir oil; however, it is preferred to. employ aliquid which is miscble with both the reservoir oil andthe displacementfluid to obtain a more'complete recovery of oil'. When water orA ahydrocarbon gas 'is employed as the displacementflluid,

such miscibility can be achieved by use of solvents mis-4 cible-withwater or the hydrocarbon'gas and reservoir' oil, e.g.,any of theaforementioned' oxygenatedl come pounds,` `and other solvents asacetonitrile, a'crylonitrle. carbon tetrachloride, methylchloride,`sulfur dioxide, and dchloro-ditluoromethane. relatively low viscosity,various viscosity additives must' be employed in admixture withthem,eg., sugars, gums, resins. and glucosides, can be added ,to increase thesolv'. j ent viscosity. Examples of gum and resin .solutionsare-r gumbenzoin, rosin. ester gum, or sandarak in acetone.

gum benzoin, elemi, ester gum, gum mastic, kauri, 'rosinz or sandarak inphenol, normal and sobutyl alcoholfn-l butyl glycol, cyclohexanone.ethyl ether, ethylglycol. methyl alcohol, methyl glycol, methylenechloride, and* An example of'a crudel sugar in a solvent is fructose inalcohol or acetone, while normal and isopropyl alcohol.

various glucosides, suchas absenthin, aesculin, antiarin,

cerberin, convallamarin, coriantyrtin, curangin, digitalim. v

digitonin, digitoxin, glucogallin, glycovanillin, glycyphyl- "viscousliquid "of the Because Ythese liquids have 7 and thevetin can be used inan alcohol or ether solution,

such as methyl, ethyl, normal and isopropyl alcohol, or dimethyl ether,diethyl ether, etc.

The invention can also be practiced by use of water as the viscousliquid when the reservoir oil and th: displacement lluid have aviscosity' less than the injected water. Frequently, however, thereservoir oil has a viscosity greater than water and in these cases aswell as those cases in which water is used as the displacement lluid, itis within the scope ot this invention lo add various thickening agentsto form a viscous aqueous solution for use as the viscous liquid.Suitable agents are crude sugars, sucrose, fructose, galactose, etc.;the afore- -mentioned glucosides; fatty acid soaps and metal alginatcs;as well as various water-soluble polymers such as carboxymethylcellulose, polyvinyl and polyallyl alcohols, and partially hydrolyzedpolyacrylamides.

Suitable partially hydrolyzed acrylamide polymers are water-solubleacrylamide polymers which have been hy- -drolyzed to such an extent thatbetween about 0.8 and l0 percent of the amide groups have been convertedto carboxyl groups. As herein employed, the term "acrylamide polymer isinclusive ot' the homopolymers of acrylamide, i.e., polyacrylamide, andwater-soluble copolymers. of acrylamide with up to about l5. percent by.weight of other polymerizable vinyl compounds; such as the alkyl estersof acrylic andmethacrylic acids, methacrylamide, styrene, vinyl acetate,acrylonitrile, methacrylonitrile, vinyl alkyl ethers, vinyl chloride,vinylidene chloride. etc.. In addition to the aforementioned limitationon the extent of hydrolysis, thc suitable acrylamide polymers are ofsulliciently` high molecular weight that a 0.5 percent by weight.aqueous solution thereof has a-.viscosity-of atleast 4centipoiscsOstwald at 21.5 C. A commercially available-polymer meetingthese-requirements is marketed by'The Dow Chemical Company under thename Scparan." The abovementioned polymers are preferred for an aqueousviscous liquid, since they form stable solutions and do not easily vprecipitate. when heated. or when in the presence of i mineral anionsand cations; however, .the use of any other [n] :KMa where K and aareconstants for a given series of polymers; M equals the polymer molecularweight; n is the intrinsic viscosity and is obtained by extrapolating tozero concentration a plot of:

versus. concentration, where C=concentration; n=viscosity of solution;fr0-:viscosity of solvent.

The values of K and a are reported in available literature on polymers,e.g., Synthetic Rubber, by G. S. Whitby,

Any long-chain oil-soluble polymer exhibiting a high solution viscosityis appropriate 'for use in accordance with the invention. Examples ofsuitable polymers are:

j polymers' of butadiene, isobutylene, chloroprene, co-

U polymers of butadiene and styrene, methyl-mcthacrylate, oracrylonitrile, copolymers of isobutylene and isoprenc, natural rubber,and polyvinyl chloride plasticized with tricresyl phosphate. The aboveformulas show that the iireaseimviscosity of a polymer solution is adirect function of the polymer molecular weight. Expressed in anotherform, the formulas show that the greater thc polymer molecular weight,the lower the concentration of the polymer needed to achieve any givensolution viscosity. The maximum molecular weight of polymers useful inthis invention is limited only by-the solubility of the polymer in themiscibledisplacement lluid. Although the lower molecular weightpolymers-are more soluble, their use is disadvantageous since relativelylarge quantities of the polymer must be added to achieve a satisfactoryviscosity increase. The preferred polymers are those which elfcct asubstantial increase in the viscosity of the solution at concentrationless than about l percent by weight, and which have molecular weightsgreater than about 10,000 and preferably between about 500,000 and1,500,000. Suicient polymer is added vto the hydrocarbon liquid to causemore than a l() percent increase in viscosity. The amount of polymeradded varies for each polymer but generally between 0.0l and l0 percentby weight, and preferably from 0.05 to 5 percent b vweight. Thehydrocarbon solvent can be any liquid in which the polymer can bedissolved and which preferably,` but not necessarily, is -miseible withthe reservoir oil and the displacement fluid. Examples of solventsinclude petroleum distillatcs, crude oil, petroleum ether, paratlinhydrocarbons such as propane, butanc. pentane, hexane, heptane, LPG.etc.; amm-aries. such as xylcne toluene, benzene, coal tar oils`cyclohexane, or mixtures thereof. Preferred fluids are LPG,

' propane, butano, isobutane, pentane. isopentane. hexane,

isohcxane. gasoline and kerosene. The optimum combination of polymer andsolvent will` ofcourse, be dictated by reservoir characteristics,economic considerations and extent of elongation of the displacing uidfront.

The invention has particular advantage when applied to a miscible uiddisplacement process employing a miscible slug comprising ahydrocarbon-polymer solution. ln this process, a long chain oil-solublepolymer, for example. polyisobutylene. is dissolved in a low molecularweight hydrocarbon solvent such as pentanc, and

a suflicient amount of the solution is injected into the reservoir toinsure separation between the reservoir Oil and a subsequently injectedgaseous drive uid. Ordinarily, the use of low molecular weighthydrocarbons in this manner-.is somewhat unsatisfactory from thestandpoint of cost. since the suitable hydrocarbons are eitherrelatively expensive per se or must be used with a.

relatively expensive sccondarv solvent in order to dissolve an effectiveamount of the polymer.

The process of the present invention. however. permits the use ofinexpensive lower members of the aliphatic hydrocarbon series` eg., LPG.propane. or butano, as the miscible hydrocarbon liquid without the needof an expensive secondary solvent to dissolve the polymer. This isaccomplished in accordance with the following procedue: A suicientquantity of LPG is iniected into the reservoir to insure separation ofthe reservoir oil and a subsequently injected. gaseous drive uid, e.g.methane. This reouires from about 3 to l5 volume percent of thereservoir pore volume. and is dependent upon the mobility ratio betweenthe LPG and reservoir oil, being about 3 percent for a mobility ratio ofone or less and about l0 percent for a mobility ratio of about 50.

. 7- dncing well itwill have been enriched with the reservoir oil.Although the LPG slug originally injected into the reservoir had verylittle solubility for a viscous long chain polymer, the slug as itreaches the producing well, enriched with oil, has a high solubility forsuch a polymer.

A concentrated solution of polyisobutylene in a solvent such as pentane,benzene, or crude oil can, therefore, bc injected into this well beforeit is closed and the polymer will dissolve in the -slug to provide avcry viscous region adjacent the tingcr. A third weil is then opened tocontinue the advancement of the slug in thc same general direction, andafter the slug has completely swept past the closed second well, thiswell can be switched to injection of methane, and the first well can beclosed. To insure that the polymer solution is injected from the secondwell directly into the finger, the trird well can be opened toproduction and the second well temporarily closed prior to polymerinjection so as to draw the nger beneath the second well. Thereafter,the second well is opened and the polymer solution injected directlyinto the nger. Because of the local high viscosity of the slug at itsmost advanced or fingered position, the fingering tendency iseffectively corrected and the miscible uid advances towards the thirdwell more evenly. This same procedure can be followed along a continuousrow of wells in a field, i.e., a rst well or row of wells can beemployed for injection, and a second, third, fourth, etc., row of wellsare then employed in the following sequence of steps: (l) withdrawal;(2) polymer solution injection; (3) closed in; and (4) gas driveinjection; until the miscible slug phase has swept through the entirereservoir.

The invention will be further described by the following examples. whichare for illustration only and are not to be considered as limiting theinvention:

EXAMPLE I -A consolidated sandstone plate, with a porosity of about 30percent, one-eighth inch thick, 2l inches Wide and 7 inches long, wassealed between two plates of Lucire. Two rows of 7 holes each weredrilled into one of the Lucite plates, equally spaced along the lengthof the plate adjacent each of the longer edges. A third row of similarlyspaced holes was drilled along the mid-line of the plate to divide theplate into twelve squares, three and one-half by three and one-halfinches. These squares are shown by the faint grid lines of FIG- URES 2,3, 4.and 5. which also show the numbering of the holes from left toright, from the upper row tothe lower row. -Tubing connections wereinserted into the holes to permit passage or" lluids through thesandstone model. The latter was then saturated with clear keroseriehaving a viscosity of 1.76 centipoises at room temperature (75 E). Themodei was held in a horizontal plane above a light source to permitobservance of the ow pattern during the run. A run typical ofconventional practice was commenced by injecting a red displacing iluid,consisting principally of hexane and having a viscosity of 0.32ccntipoises at roorn temperature, into well 18 whileY withdrawing clearkerosene from well 19. This injection was continued until a reddisplacing tluid nger broke through to well 19, whereupon wells 18 and19' were closed and the iiow pattern was traced onto a sheet of clearplastic placed over the model. This tracing is shown in FIGURE 2, asfiuid front F-1. The run was continued by repeating the injection of thedisplacing tluid into well 18 (well 19 being closed) and withdrawingfrom well 20 until the displacing duid broke through to well 20. The owpattern was traced and is shown in FIGURE 2 by iluid front F-2. Thisprocedute was repeated, employing wells 21 and. 22 successively aswithdrawal wells and continuing injection into well 18. The fluid frontsat breakthrough to wells 21 and 22 are shown by F-S and F-4,respectively. During the run the displaced kerosene volume wasmeaslCmivcntional lisplaceuient-volunie kerosene displaced atbreakthrough 1 Well: Total, milliliters 19 2.6 20 7.6 21 13.2A

EXAMPLE n The sandstone model was tlushcd clean and again saturated withclear kerosene, and the procedure of Example l was repeated. againemploying red hexane as a displacing fluid. Viscosities of the keroseneand hexane were the same as in Example'l. The displacing iluid front atbreakthrough to well 19 is shown as F-S and at break through to well 20as F-6 in FIGURE 3. At this point the invention was applied by theinjection of a mixture of oil andikerosene having a viscosity of 5centipoises into well 20 with wells 13 and 27 open to withdraw thedisplaced kerosene. A blue dye was added to this viscous liquid toidentify it during the test.l The injected slug of viscous liquid afterinjection is shown by the shaded area surrounding wcll 20. Duringinjection of this viscous slug, no noticeable shifting of tluid front,F-6, occurred. After two milliliters of the viscous uid had beeninjected into well 20, wells 13, 20 and 27 were closed, well 18 wasreopened to injection of the hexane displacing fluid, and well 21 wasopened to withdrawal of kerosene. After breakthrough to well 21 of theviscous liquid slug, wells 18 and 21 were closed and the tlow patternswere traced to obtain FIGURE 4 which shows the displacing iuid front asF-7 and the viscous liquid slug as the shaded area. Thereafter,injection into well 18 was repeated, with well l22 opened for withdrawalof kerosene.` The displacing uid and viscous liquid slug both lingeredinto well 22, at which point wells 1S and 22 were closed, and the howpattern was traced to obtain FIGURE-5. The uid front of the displacinguid is shown as F-S and the viscous liquid slug as the shaded area in'FIGURE 5. The displacing tluid and viscous liquid mixed along theirinterface, this being indicated in FIGURE 5 by the less dense shading ofthe trailing portion of the viscous slug. As in Example I, the volume ofdisplaced kerosene was measured and recorded at breakthrough to eachwell. This data appearsAv in the following table:

Table 2 [Typical of the invention-volume kerosene displaced atbreakthrough] Well: l Total, milliliters 19 2.4

*Does no t include the two milliliters withdrawn fromx wells 13 nnu 2Tduring injection of the viscous liquid slug.

A visual'comparison of FIGURES 2 and 5 shows that the total area sweptby the displacing duid and viscous slug in FIGURE 5 is much greater thanobtained without use of viscous liquid injection as shown in FIG- URE 2.In FIGURE 2 it can'be seen that the tiuid front rapidly narrowed intoa'tinger extending past wells 20 and 21, to well 22. A comparison or'the amount of kerosene displaced shows that even when disregarding thetwo milliliters displaced by injection of the viscous liquid slug, thetotal recovery of Example II is 28.9

percent greater than that of Example I. The actual recovery, includingthe two milliliters produced through wells 13 and 27, is 40 percentgreater.

The invention has been illustrated in the foregoing examples under idealreservoir conditions, i.e., with a reservoir having a uniformpermeability throughout, and

with a fluid mobility ratio of displacing hexane to kerosene of1.76/0.32, or 5.5, a value usually exceeded under actual reservoirconditions. 'ihe effect of these conditionshas been to minimizefingering in the model so as to obtain tlow patterns which may bereadily evaluated. Under actual reservoir conditions, the displacing uidngering will be more pronounced and, therefore, the irnprovement of thesweep eciency of the displacing fluid when the viscous liquid slug ofthe invention fis employed will exceed that observed in the laboratorystudies.

Although the invention has been describedwith. .regard to the horizontalsweep eciency ofthe displace ment, in actual use it. can also be appliedto obtain a satisfactory vertical sweep efficiency. In this use, thevertical location of the linger is located by any suitable method, suchas by sealing off various heights of the producing borehole until theregion producing the displacement fluid is located. This zone is thensealed oi with packers and the viscous liquid slug is injected into thiszone, thereby insuring that the greatest mass of viscous liquid will belconcentrated at the exact spacial location of the nger.

The invention has. been described by reference to a displacement processbetween a single injection well` and successive withdrawal wells. Thiswas to illustrate the invention in its simplest form; in field use, aplurality of wells can be 'employed for injection and/or withdrawal. Atype dow pattern which can be expected when the invention is applied toone of .the conventional drive techniques, i.e., a line drive, is shownin FIGURE 6.

y f FIGURE. 6 illustrates a typical dow lpattern in which f ladisplacing uid which has been injected :into the reservoirv througha-rst row lof injection wells, 35, has the intermediate front shown bythe dashed lines at. .breakthrough' into a -second parallel row-ofwithdrawal wells, 49. When the lingering of the duid is pronounced asshown by FIGURE 6, slugs of viscous liquid are injected into eachproducing well. During this injection of viscous slugs, the displacedreservoir oil is lwithdrawn from the nexty succeeding parallel row ofwellsorxrom. adjacent-wellsl in the same row as the -well used to injectthe viscous slug. Alternatively, when itis desired to repressurize thereservoir, no withdrawal of reservoir oil is made. When the wells areclosely spaced and the injected liquid is not. extremely viscous, theshape of the injected viscous slug can readily be controlled byfproperchoice Lof withdrawal vwells during injection. This is l illustrated inFIGURE 6 bythe .various shaped areas surrounding wells 41, -42and 43.The slug injected into well 4l illustrates a zone having acircularhorizontal cross section, typical of vinjection into a reservoirwhen withdrawal is from a well or wells spaced a substantial distance.from that employed vto inject the viscous slug and/or when an extremelyviscous liquid is injected into a. reservoir, or whenY no withdrawal ofreservoir oil .is made. The elliptical shaped slug at well 42 representsinjection of a relatively low viscosity. liquid into the reservoir whilethe displaced vreservoir oil is withdrawn from adjacent wells 41 and 43.This method is preferred since it distributes the greatest portion ofthe slug transverse to the direction of lingering. Well 43 is shown witha slug having a circular horizontal cross section obtained by viscousliquid injection without any withdrawal of reservoir oil. The amount ofslug injected into each well is directly Vproportional to the extent oflingering occurring at each well site as determined, for instance, by

the relative production from each well. It is, of course;

apparent that in the extremely rare instance where no ingerhas formedadjacent a withdrawal well, no viscous slug need be injected.

After the viscous slugs have been injected into the wells of row 40,these wells are closedand reservoir oil is withdrawn from the wells inrow 50. After the displacing uid or viscous liquid reaches the wells inthis row, they are closed to'production and reservoir oil is withdrawnfrom the next succeeding row 60, or if the extent of fingering is againpronounced, additional slugs of viscous liquid are injected through thewells in row 50. FIGURE 6 shows the iluid front as it reaches thewithdrawal wells in row and it-can be seen that substantially all thearea between rows 35, 40 and 50 has been swept by the fluid.

In another modification of the line drive method of the invention, vnotall the wells of a single row are employed for injection of the viscousslug. For instance, every second or third well in row 40 can be employedfor slug injection, such as injection through wells 4l and 43. In thesucceeding parallel row of wells 50, viscous slugs are injected throughwells opposite those wells of the preceding row which were not employedfor vis'- cous slug injection, in this case, well 52. This modificationcan suitably be used when it is desired to elongate the slug in adirection at right angles to the fluid ow direction, since wellsadjacent the well employed for slug injection are available forwithdrawal of displaced oil.

From the foregoing illustrations, it is obvious that the number andlocation of injection and/ or withdrawal wells can be varied so as toobtain substantially any desired ow pattern or sweep in a secondaryrecovery process.

Having clearly, completely and concisely described my invention, ltherefore claim:

1. The method of recovering oil from a subterranean reservoir pierced byat least three wells consisting of an input well, a second well spaced asubstantial distance therefrom and a third well located between saidinput and second wells which comprises: injecting a displacement huidinto said input well; withdrawing said oil from said reservoir throughsaid third well until the presence of said displacement lluid isdetected in the eluent from said third well; thereafter, ceasing saidwithdrawal of said oil from said third well and injecting a slug ofviscous liquid into said reservoir through said third well, said viscousliquid having a viscosity at reservoir conditions greater than theviscosity of the most viscous of said first displacement fluid and saidoil, the viscosity and amount of said viscous liquid being suicent tosubstantially retard the advance of said displacement fluid in thevicinity of said third well bore; thereafter shutting in said third welland withdrawing said oil from said second well while continuing toinject said displacement fluid into said input well.

2. The method of claim l wherein said displacement fluid is an aqueousliquid.

3. The method of claim 2 wherein said viscous liquid is an aqueoussolution of a water-soluble thickening ,agent selected from the groupconsisting of: crude sugars, glucosides, fatty acid soaps, alginates,and water-soluble polymers.

4. The method of claim 2 wherein said viscous liquid comprises a liquidwhich is miscible with said reservoir oil and with water and contains athickening agent selected from the group consisting of crude sugars,gums, resins and glucosides.

5. The method of claim 2 :1s-applied to a miscible displacement processwherein said displacement uid com`-` 8. The method of claim o whereinsaid viscous liquid comprises a liquid which is miscible with said oil,

9. The method of claim 8 wherein said viscous liquid contains athickening agent selected from the group consisting ot' long chainoil-soluble polymers, crude oil, crude oil distillates. and mixturesthereof'.y

10. The method of c1aim.9 as applied-to@ miscible displacement processwherein said displacementiluid comprises a slug of hydrocarbon which isliquid at reservoir conditions and which is first injected into saidinput .well and drive gas which is thereafter injected into said inputwell.

ll. The method of obtaining oil from a subterranean reservoir pierced bya plurality of wells substantially arranged to form at least first.second and third consecutive parallel rows oi weils which comprises:injecting a displacement lluid into said reservoir through wells in saidiirst' row; withdrawing said oil from said reservoir throughwells insaid second row; ceasing said withdrawal at each o said wells in saidsecond row when said displacement uid is present in the effluenttherefrom; in-

' jecting a viscous liquid into each of said wells of said second rowfrom which withdrawal has ceased; said viscous liquid being more viscousthan most viscous of said displacement tiuid and said oil. the viscosityand amount of said viscous liquid being suiticient to substantiallyretard the advance of said displacement uid in the vicinity of saidthird well bore; closing said wells in said second row, withdrawing oilfrom wells in said third row and continuing to inject said displacementfluid through 30 said wells in said first row.

l2. The method of claim 11'? wherein said-viscous liquid is injectedinto said reservoir throughv alternate wells in said second row, whilewithdrawal' of displaced uids from said reservoir is conducted from thewells adjacent said alternate wells in said second Arow during theinjection of said slug of viscous liquid.

13. The method of claim 11 wherein withdrawal of said oil through wellsin said'third row is initiated simultaneously with said injection ofsaid slug of said viscous liquid into said reservoir through saidwellsin said second row. i

14. The method of claim 11 wherein consecutive parallei rows of wellsare successively employed in the sequence of: first, withdrawal ofdisplaced oil; second, injection of viscous liquid; third, closed.

References Cited in the le of this patent UNITED STATES PATENTS2,771,138 Beeson Nov. 20, 1956 2,798,556 Binder et al. July'9, 19572,827,964 Sandiford et a1v Mar. 25, 1958 2,842,492 Engelhardt et al.July 8, 1958 2,920,041 Meadors Jan. 5, 1960 2,924,276 Heilman et al.Feb. 9, v1960 OTHER REFERENCES Fagin, K. M.: Effect of Well Spacing onWater Flood Operations and Economics, The Petroleum Engineer, June,1946. pages 64-72.

1. THE METHOD OF RECOVERING OIL FROM A SUBTERRANEAN RESERVOIR PIERCED BYAT LEAST THREE WELLS CONSISTING OF AN INPUT WELL, A SECOND WELL SPACED ASUBSTANTIAL DISTANCE THEREFROM AND A THIRD WELL LOCATED BETWEEN SAIDINPUT AND SECOND WELLS WHICH COMPRISES: INJECTING A DISPLACEMENT FLUIDINTO SAID INPUT WELL; WITHDRAWING SAID OIL FROM SAID RESERVOIR THROUGHSAID THIRD WELL UNTIL THE PRESENCE OF SAID DISPLACEMENT FLUID ISDETECTED IN THE EFFLUENT FROM SAID THIRD WELL; THEREAFTER, CEASING SAIDWITHDRAWAL OF SAID OIL FROM SAID THIRD WELL AND INJECTING A SLUG OFVISCOUS LIQUID INTO SAID RESERVOIR THROUGH SAID THIRD WELL, SAID VISCOUSLIQUID HAVING A VISCOSITY AT RESERVOIR CONDITIONS GREATER THAN THEVISCOSITY OF THE MOST VISCOUS OF SAID FIRST DISPLACEMENT FLUID AND SAIDOIL, THE VISCOSITY AND AMOUNT OF SAID VISCOUS LIQUID BEING SUFFICIENT TOSUBSTANTIALLY RETARD THE ADVANCE OF SAID DISPLACEMENT FLUID IN THEVICINITY OF SAID THIRD WELL BORE; THEREAFTER SHUTTING IN SAID THIRD WELLAND WITHDRAWING SAID OIL FROM SAID SECOND WELL WHILE CONTINUING TOINJECT SAID DISPLACEMENT FLUID INTO SAID INPUT WELL.